Scott H. Woodward

Holographic particle image velocimetry: from film to digital recording

Holographic particle image velocimetry (HPIV) offers potentially the best solution to volumetric measurements of the three-dimensional velocity fields in complex flows. However, the traditional film-based HPIV measurement is rather cumbersome, limiting its use to only a handful of groups worldwide. The newly emerged digital HPIV revolutionizes flow measurement science by providing a practical 3D velocimetry tool. It commands simple hardware that is similar to regular two-dimensional particle image velocimetry (PIV), yet it provides continuous (time-series) three-dimensional, three-component flow field data. Not only is the need for chemical processing eliminated, but also the cumbersome optical reconstruction is completely replaced by numerical reconstruction algorithms. Several breakthroughs have led to the development of the first practical and integrated digital HPIV systems. To explain the transition from film to digital recording, fundamental issues in HPIV are reviewed in this paper. Axial accuracy in HPIV measurement is ultimately limited by an inherent depth-of-focus problem, while information capacity is limited by inherent speckle noise. Information capacity is an important concept in HPIV, comprising the maximum acceptable seeding density multiplied by the sample volume depth along the optic axis. Both the axial accuracy and the information capacity are limited by the effective hologram aperture. The pursuit of a large hologram aperture in the past has resulted in further complexity in film-based HPIV systems. Digital HPIV, on the other hand, enjoys great simplicity of implementation and operation. A digital HPIV is also far more compact and rugged compared to existing film-based HPIV systems, making it suitable for duplication and commercialization. However, since digital sensors suffer from inferior pixel resolutions compared to films, the effective hologram aperture is much smaller in digital HPIV than that achievable in film-based HPIV. Alleviating this problem, digital HPIV also presents new possibilities in data processing such as the use of the complex amplitude of the reconstructed light wave to improve depth sensitivity and signal-to-noise ratio. Two examples of digital HPIV systems and measurement results are given. We believe digital HPIV can revitalize holographic particle imaging and bring it into the mainstream in much the same way that digital PIV brought PIV into widespread use a decade ago.

Validation of CFD Simulations of Cerebral Aneurysms With Implication of Geometric Variations

BACKGROUND Computational fluid dynamics (CFD) simulations using medical-image-based anatomical vascular geometry are now gaining clinical relevance. This study aimed at validating the CFD methodology for studying cerebral aneurysms by using particle image velocimetry (PIV) measurements, with a focus on the effects of small geometric variations in aneurysm models on the flow dynamics obtained with CFD. METHOD OF APPROACH An experimental phantom was fabricated out of silicone elastomer to best mimic a spherical aneurysm model. PIV measurements were obtained from the phantom and compared with the CFD results from an ideal spherical aneurysm model (S1). These measurements were also compared with CFD results, based on the geometry reconstructed from three-dimensional images of the experimental phantom. We further performed CFD analysis on two geometric variations, S2 and S3, of the phantom to investigate the effects of small geometric variations on the aneurysmal flow field. Results. We found poor agreement between the CFD results from the ideal spherical aneurysm model and the PIV measurements from the phantom, including inconsistent secondary flow patterns. The CFD results based on the actual phantom geometry, however, matched well with the PIV measurements. CFD of models S2 and S3 produced qualitatively similar flow fields to that of the phantom but quantitatively significant changes in key hemodynamic parameters such as vorticity, positive circulation, and wall shear stress. CONCLUSION CFD simulation results can closely match experimental measurements as long as both are performed on the same model geometry. Small geometric variations on the aneurysm model can significantly alter the flow-field and key hemodynamic parameters. Since medical images are subjected to geometric uncertainties, image-based patient-specific CFD results must be carefully scrutinized before providing clinical feedback.

Mathematical model of the rupture mechanism of intracranial saccular aneurysms through daughter aneurysm formation and growth

Abstract Objectives: Daughter aneurysms have been strongly associated with saccular aneurysm rupture. We constructed a mathematical model to help explain this association as a possible hemodynamic mechanism for intracranial saccular aneurysm rupture. Methods: Our model is based on the assumption that when an aneurysm reaches a state of imminent rupture, the weakest area of the aneurysm wall responds passively to a surge of intra-aneurysmal pressure by forming a daughter aneurysm that will be the site of the eventual rupture. The daughter and parent aneurysms were assumed to be spherical. Using mathematical modeling, the growth of the daughter aneurysm was observed. To obtain the change in tensile stress in the daughter aneurysm wall under constant pressure and changing geometry, the Law of Laplace was applied to the parent and the daughter aneurysms. Results: The model reveals that the stress factor, i.e. tensile stress in the daughter aneurysm wall relative to the wall strength (rupture point), is dependent on two geometric parameters: the orifice factor (μ), which represents the relative size of the daughter aneurysm orifice radius to the parent aneurysm radius; and the aspect ratio (λ), which represents the height-to-orifice ratio of the daughter aneurysm. As the daughter aneurysm develops, the stress factor first decreases to protect against rupture. Minimal stress is attained at an aspect ratio (λ) of 0.577 regardless of the orifice factor. This is a relatively stable state. Further growth of the daughter aneurysm results in an increase of stress above the minimum, eventually leading to rupture at a stress factor of 1. A smaller orifice factor μ allows this aneurysm to grow to a higher aspect ratio λ before rupture. Discussion: Daughter aneurysm formation is a likely path to aneurysm rupture. The formation of a daughter aneurysm temporarily decreases the tensile stress within a parent aneurysm in which rupture is imminent, indicating a temporary protective role of daughter aneurysm development. Aneurysms harboring daughter aneurysms are at a more advanced stage of development, hence at a greater risk for rupture. The severity of the rupture risk can be estimated on the basis of daughter aneurysm geometry; aspect ratio λ> 0.577 indicates a greater risk of rupture. Furthermore, daughter aneurysms with larger orifices are associated with a greater risk of rupture.

Proper orthogonal decomposition of an axisymmetric turbulent wake behind a disk

A proper orthogonal decomposition (POD) study of the axisymmetric turbulent wake behind a disk has been performed using multipoint hot-wire data. The Reynolds number based on the free stream velocity and disk diameter was kept constant at 28 000. The investigated region spanned from 10 to 50 disk diameters downstream. The hot-wire data were obtained using two rakes: a seven wire fixed array and a six wire array azimuthally traversable to span the cross section of the flow in increments of 15°. The instantaneous streamwise velocity component data were Fourier transformed in time and decomposed in Fourier series in the azimuthal direction to form the kernel for the POD. For all downstream positions, two distinct peaks were found in the first eigenspectrum: one at azimuthal mode 2 at near zero frequency, and another at azimuthal mode 1 at a fixed Strouhal number (fd/U∞) of 0.126. Both peaks decrease in magnitude as the flow evolves downstream, but the peak at the Strouhal number 0.126 decreases more rapidly ...

An Evaluation of Analog and Numerical Techniques for Unsteady Heat Transfer Measurement with Thin-Film Gauges in Transient Facilities

Abstract The importance of frequency response considerations in the use of thin-film gauges for unsteady heat transfer measurements in transient facilities is considered, and methods for evaluating it are proposed. A departure frequency response function is introduced and illustrated by an existing analog circuit. A Fresnel integral temperature which possesses the essential features of the film temperature in transient facilities is introduced and is used to evaluate two numerical algorithms. Finally, criteria are proposed for the use of finite-difference algorithms for the calculation of the unsteady heat flux from a sampled temperature signal.

Application of a ''slice'' proper orthogonal decomposition to the far field of an axisymmetric turbulent jet

Instantaneous measurements of the streamwise velocity component were obtained in the far field region of an axisymmetric turbulent jet at exit Reynolds numbers ranging from 40 000 to 84 700. The data were taken from 20 to 69 diameters downstream of the jet exit using 138 hot-wires. The proper orthogonal decomposition was applied to a double Fourier transform in time and azimuthal direction of the two-point velocity correlation tensor. The first eigenspectrum, which contains more than 60% of the kinetic energy, has two peaks: A dominant peak at azimuthal mode-2 at near zero frequency, and a secondary peak at mode-1 at a local Strouhal number (fx/Uc) of approximately 1. The most striking feature was the general behavior of the eigenspectra: when normalized to reflect the energy repartition per mode number only, they peaked at mode-2, and were independent of downstream position in the experiment considered. They were also similar in nature to those obtained in earlier experiments at x/D=6, just past the end ...

Hybrid digital holographic imaging system for three-dimensional dense particle field measurement

To apply digital holography to the measurement of three-dimensional dense particle fields in large facilities, we have developed a hybrid digital holographic particle-imaging system. The technique combines the advantages of off-axis (side) scattering in suppressing speckle noise and on-axis (in-line) recording in lowering the digital sensor resolution requirement. A camera lens is attached to the digital sensor to compensate for the weak object wave from side scattering over a large recording distance. A simple numerical reconstruction algorithm is developed for holograms recorded with a lens without requiring complex and impractical mathematical corrections. We analyze the effect of image sensor resolution and off-axis angle on system performance and quantify the particle positioning accuracy of the system. The holographic system is successfully applied to the study of inertial particle clustering in isotropic turbulence.

Flow modification in canine intracranial aneurysm model by an asymmetric stent: studies using digital subtraction angiography (DSA) and image-based computational fluid dynamics (CFD) analyses

An asymmetric stent with low porosity patch across the intracranial aneurysm neck and high porosity elsewhere is designed to modify the flow to result in thrombogenesis and occlusion of the aneurysm and yet to reduce the possibility of also occluding adjacent perforator vessels. The purposes of this study are to evaluate the flow field induced by an asymmetric stent using both numerical and digital subtraction angiography (DSA) methods and to quantify the flow dynamics of an asymmetric stent in an in vivo aneurysm model. We created a vein-pouch aneurysm model on the canine carotid artery. An asymmetric stent was implanted at the aneurysm, with 25% porosity across the aneurysm neck and 80% porosity elsewhere. The aneurysm geometry, before and after stent implantation, was acquired using cone beam CT and reconstructed for computational fluid dynamics (CFD) analysis. Both steady-state and pulsatile flow conditions using the measured waveforms from the aneurysm model were studied. To reduce computational costs, we modeled the asymmetric stent effect by specifying a pressure drop over the layer across the aneurysm orifice where the low porosity patch was located. From the CFD results, we found the asymmetric stent reduced the inflow into the aneurysm by 51%, and appeared to create a stasis-like environment which favors thrombus formation. The DSA sequences also showed substantial flow reduction into the aneurysm. Asymmetric stents may be a viable image guided intervention for treating intracranial aneurysms with desired flow modification features.

Downstream Evolution of the Most Energetic POD Modes in the Mixing Layer of a High Reynolds Number Axisymmetric Jet

Experimental results were obtained in the potential core region of an axisymmetric turbulent jet from 2 to 6 diameters downstream, at Reynolds numbers of 78, 400, 117, 600, and 156, 800. Data were collected using the 138 hot-wire probe used by Citriniti and George (2000). The Proper Orthogonal Decomposition was then applied to a double Fourier transform in time and azimuthal direction of the double velocity correlation tensor. The lowest azimuthal mode for all POD modes, which dominated the dynamics at x/D = 3 in the previous experiments, dies off rapidly downstream. This is consistent with a trend toward homogeneity in the downstream evolution, and suggests that some residual value may control the growth rate of the far jet. On the other hand, for the higher azimuthal modes, the peak shifts to lower mode numbers and actually increases with downstream distance. These mixing layer data, normalized by similarity variables for the mixing layer, collapse at all downstream positions and are nearly independent of Reynolds numbers.

Qualitative and quantitative wind tunnel measurements of the airflow through a shrouded airborne aerosol sampling probe

In this paper, we present the results of a series of wind tunnel tests of the airflow through a shrouded probe designed by Ram et al. (1995, J. Aerosol. Sci. 26, 945–962) for high velocity airborne sampling of insoluble aerosol particles in the size range 0.1–15 μm diameter. Tests included flow visualization using a laser light-sheet and hot wire anemometry measurements of air velocity profiles within the shroud upstream of the sampling probe with the probe operating at angles of attack from 0 to 15°. We discuss the effectiveness of the shroud in making the flow into the probe isoaxial for situations where the sampler is at small angles of attack and comment on the role of the shroud as an effective flow decelerator. We conclude by comparing the wind tunnel measurements with numerical simulation studies of the airflow through the sampler at 0° angle of attack.